The existing ceiling joists are unlikely to be adequate to take a conversion floor, so additional new joists will be required to comply with the Building Regulations. The size and grade would have been specified by the structural engineer, who will have taken into account the span and the separation distance for a given loading. The new joists span between load-bearing walls, and are normally raised slightly above the existing ceiling plasterwork by using spacers below the joist ends. This spacing must be sufficient to prevent any new floor joist deflection from touching the ceiling plaster below. The new joists run alongside the existing joists. Above window and door openings, thicker timbers are used to bridge the opening, so that pressure is not put on the existing opening lintel. RSJs are also specified to distribute the load, and in some installations are used to carry the ends of the new joists. If head height is limited, then thicker joists, more closely spaced, can be specified.
Your Building Control inspector will specify exactly what you require. The roof structure can be insulated in one of two main ways. The most straightforward is to use a ‘cold roof’ method. This involves filling the space between the rafters with 70mm-thick slab foam insulation such as Celotex, ensuring that there is 50mm spacing between the roofing felt and the insulation (for ventilation via the roof and soffit vents). In addition, 30mm slab insulation is attached to the inside of the rafters, giving a total of 100mm of insulation. The rafter thickness is often less than 120mm, so a batten may be required along each rafter to allow the 50mm spacing and the 70mm insulation. The roof section requires 300mm of mineral wool insulation (e.g. Rockwool), or 150mm of slab foam insulation, such as Celotex. This method can be undertaken by the DIYer.
The other main method is ‘warm roof’. This method uses 100mm Celotex insulation or similar over the rafters, and a covering capping, followed by the tile battens and tiles. This is not really a practical option unless the roof coverings have been stripped off. It could be used with a dormer, especially if it has a flat roof. Continuity of insulation between walls and roof is required to avoid any cold bridging. The dormer walls can be insulated with 100mm Celotex between the studwork. The internal partition walls use a 100mm quilt that will provide sound insulation. Plaster – board is attached to one side of the wall then the quilt inserted, followed by plaster – board on the other side. Insulation is also placed between floor joists, and this is typically 100mm-thick Rockwool fibre or similar — mainly for its sound-reduction properties.
The ideal location for a staircase to land is in line with the roof ridge: this will make best use of the available height above the staircase. The minimum height requirement above the pitch line is 2m, although this could be reduced to 1.9m in the centre, and 1.8m to the side of a stair. In practice, the actual position will depend upon the layout of the floor below, and where necessary the available height can be achieved using a dormer or adding a rooflight above the staircase or, if appropriate, converting a hip roof end to a gable.
Maximum Number of Steps: The Regulations specify that the maximum number of steps in a straight line is 16. This is not normally a problem, as a typical installation usually only requires 13 steps.
Step Size: The maximum step rise is 220mm, whereas the step depth or ‘going’ is a minimum of 220mm; these measurements are taken from the pitch point. The step normally has a nose that projects 16-20mm in front of the pitch line. However, the ratio of size must not exceed the maximum angle of pitch requirement of 42°. Any winders must have a minimum of 50mm at the narrowest point. The width of steps is unregulated, but in practice the winders are likely to limit the reduction in width.
The height minimum is 900mm above the pitch line, and any spindles must have a separation distance that a 100mm sphere cannot pass through.
Windows & Dormers
The loft conversion will require a means of getting natural light and ventilation, and the most straightforward method is to use rooflights that follow the pitch line of the roof. This type is fitted by removing the tiles and battens in the position that the rooflight will be fitted. The rafters are cut to make way for the rooflight after suitably reinforcing the remaining rafters. The rooflight frame is then fitted within the new opening, and flashings added before making good the surrounding tiling. This type of window is the most economic, and more likely to be allowed without planning permission, under your PD (Permitted Development) rights. Conservation rooflights, which are slightly more flush to the roofline and are made of metal, can also be specified.
Dormers not only give natural light but can add space to a loft conversion; they can be at the ends or sides. They are particularly effective where the pitch angle is high, as the useful floor area can be increased. The mansard type (ABOVE) will give maximum conversion roof space because it projects the maximum available head height, thus giving a greater usable floor area. A hip to gable conversion has a similar effect.
Dormers and other similar conversions are normally installed by opening up the roof, and cutting the required specified timbers to size on site. They normally involve compound angle cuts (SEE BELOW) so may not be a task that a DIYer would like to undertake. Care also needs to be taken with the roof and side coverings, to get a good weatherproof structure.
Some loft conversion companies will make the dormers off site in their workshop and lift into place. This process allows quick installation, and quick weatherproofing.
Dormers can have gabled or hip roofs, and with careful design can enhance a roof line. In practice, a mixture of the available types can result in the maximum light and space, and provide a fire exit.
The plasterboard ceiling in the upper rooms will delay the spread of fire to the roof space in an unconverted house. However, when an opening is introduced for the staircase the risk is shared with the conversion — therefore, safeguards must be in place to reduce the risk.
All habitable rooms in the upper storeys served by a single staircase should have an escape window with an obstructed openable area of at least 0.33m², a minimum 450mm high x 450mm wide, and not more than 1.1m above the floor level. For loft conversions to existing two storey houses, more stringent provisions apply, due to the greater risk associated with escape via high-level windows. These require a new 30- minute fire-resistant floor to the loft conversion, and a protected 30- minute fire-resistant stair enclosure discharging to its own final exit, with fire doors to all rooms (except bathrooms and WC). The fire doors do not need to be self-closing.
At least one mains-operated smoke alarm with battery backup must be installed in the circulation space of each storey. All alarms are to be interconnected.